The natural gas liquefaction processes currently employed in base load LNG plants include the propane pre-cooled mixed refrigerant process developed by Air Products and Chemicals, Inc. of the United States, and the TEALARC process developed by Technip of France. However, in both cases, either propane or a mixture of propane and ethane is used for the pre-cooling of the natural gas (to approximately -40.degree. C.), and the final cooling step (from -140.degree. C. to -160.degree. C.) is carried out with a refrigeration cycle of a mixed refrigerant (a mixture of nitrogen, methane, ethane and propane) using a huge Humpson type heat exchanger. In a Humpson heat exchanger, a multiplicity of turns of aluminum tube are wound around a core rod, and a LNG plant with an annual output of 1.0 million tons typically requires a huge Humpson type heat exchanger which is 50 m tall, weighing 100 tons.
Such a heat exchanger is extremely heavy due to its structural features. Further, since an extremely long time is required to have such a heat exchanger fabricated and fabrication requires a plant equipped with special facilities for complicated fabrication processes, the cost for constructing a LNG plant is thereby increased, especially for small or off-Shore LNG plants.